Someone who's been in too many fistfights and thus has had the ability to breathe through the nose beaten out of him. The usual implication is that the ability to use the brain has also been beaten out of him.

My bookie sent a mouth breather to collect for him, but when the goon tried to get me to head to the ATM with him I escaped for the moment by saying his shoe was untied.

1. A moron from a low social caste. Picture someone with a sunken forehead, low hairline, and prominent chin.

2. A guy who goes to the gym seven days a week, probably does steroids, and has a body that looks like a comic-book character. He might be 5'4" but his arms and shoulders are so overbuilt that he looks ridiculous.

If you go to Gold's Gym in the afternoon there are nothing but mouth-breathers.

1. A term used by intolerant people or by the elite, who find some need, probably from a lack of self esteem, to marginalize someone whose opinion differs from their own. Usually they would define a mouth breather as someone who is to stupid to "get it". A a socially backward individual.

2. Someone who suffers from allergies or other condition that prevents breathing through the nose.

These days, someone who supports President Bush or the NRA, would be described by liberals as a mouth-breather.